creating amending moving grub bootloader

I need to create a new GRUB bootloader. My PC originally had two hard drives – one containing Win 98 (used by my wife!), a “Shared” partition accessible to both Linux and Windows together with a current laboriously updated Ubuntu system. The other small drive has another copy of Win 98 which is used solely to automatically back up “Win98/My Documents” from the main drive. (I have always insisted on a separate back up drive.) At some stage when I was “experimenting” with the Ubuntu installation (and after downloading a fair number of updates) I made a boob and lost/corrupted the bootloader. I decided an easy solution (since Ubuntu installs so quickly) was to introduce a third small drive and install Ubuntu on it SOLELY to create a new bootloader which would “pick up” the other operating systems. (The beginner's approach!)
This works well and when I start I can choose which system to boot from with the normal menu. However I now need to remove the third drive (to replace it with a DVD drive) and hence my problem. If I disconnect this drive GRUB comes up with error code 21 (missing device) as you would expect.
How do I create a new/amended GRUB – which would have to be installed onto the main drive – so that I can once again boot from Ubuntu/Windows1/Windows2? I do NOT want to have to re-install Ubuntu from scratch – downloading all those updates again with a dialup connection would give me a coronary!! Any advice would be very welcome.